1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) element, and also, relates to a light emitting apparatus, an image forming apparatus, a display apparatus, and an imaging apparatus each using the organic EL element.
2. Description of the Related Art
In recent years, an organic EL element that emits light spontaneously with a low driving voltage of about several volts is drawing attention. The organic EL element has a construction in which a reflecting electrode having a metal reflective layer, an emission layer, and a transparent electrode are laminated. Due to excellent features such as surface emitting characteristics, light weight, and visibility the organic EL element is being put into practical use as a light emitting apparatus of a thin display, lighting equipment, a head-mounted display, or a light source for a printhead of an electrophotographic printer.
Along with a demand for low power consumption of a display apparatus constructed using the organic EL element, improvement of luminous efficiency of the organic EL element is being expected. One of element structures improving the luminous efficiency remarkably is a micro cavity system. Light emitting molecules have a feature of radiating light strongly toward a space in which “enhancing interference” of light occurs. Specifically, the radiation rate of excitons can be increased and the radiation pattern thereof can be controlled through use of optical interference. According to the micro cavity system, element parameters (film thickness and refractive index) are designed so that the “enhancing interference” occurs in a light-extraction direction viewed from light emitting molecules. In particular, it is known that, in the case where a distance d0 between the metal reflective layer and the emission layer satisfies the condition: d0=λ/(4n0) (hereinafter, referred to as λ/4 interference condition), a radiation intensity is increased most by an interference effect. Here, λ indicates a peak wavelength (in a vacuum) of a PL spectrum of light emitting molecules, and n0 corresponds to an effective refractive index between a light emitting point and a metal reflective layer. According to the micro cavity system, it is not necessary to use an uneven structure such as a microlens, and an increase in the luminous efficiency at low cost can be expected.
In addition, a micro cavity is classified into a weak micro cavity and a strong micro cavity depending on the magnitude of a reflectance on a light-extraction side. Generally, in the weak cavity, an electrode structure having a high transmittance such as a glass/transparent oxide semiconductor is used, and an interference effect of the cavity is determined mainly by an interference condition between a metal reflective layer and the emission layer. On the other hand, in the strong cavity, a semi-transparent metal thin film having a high reflectance is used as a transparent electrode on the light-extraction side. Therefore, the strong cavity includes not only an interference effect obtained between the metal reflective layer and the emission layer but also the interference effect obtained between the emission layer and a metal thin film on the light-extraction side. In this case, an optical distance between the emission layer and the metal thin film on the light-extraction side is also designed so as to satisfy the λ/4 interference condition in such a manner that the interference effect becomes maximum. Therefore, in the strong cavity, the interference effect larger than that in the weak cavity can be used, and thus, the luminous efficiency can be improved remarkably.
However, it is known that, in the λ/4 interference condition, the distance between the emission layer and the metal reflective layer is about 60 nm or less, and hence surface plasmon (SP) loss becomes large. The SP loss is a phenomenon in which an SP of metal is excited by excitation energy of light emitting molecules, and as a result, the excitation energy is transformed into Joule heat. Therefore, the micro cavity using the λ/4 interference structure has a problem in that the luminous efficiency is not improved with respect to a large optical interference effect. Specifically, in order to further improve the luminous efficiency of the micro cavity under the λ/4 interference condition, a method of suppressing the SP loss is required.
Hitherto, as the method of suppressing the SP loss, a method of sacrificing the interference effect such as increasing the distance between the metal reflective layer and the emission layer (Japanese Patent Application Laid-Open No. 2008-543074) has been proposed. In recent studies, a method of satisfying both the interference effect of λ/4 and suppression of the SP loss, such as orienting a transition dipole moment of light emitting molecules horizontally (J. Frischeisen et al., Organic Electronics 12, (2011), 809-817) has started being proposed. Each of the proposals for suppressing the SP loss introduced in the foregoing has been investigated in a weak cavity construction having only one interface between a metal and a dielectric. In other words, no proposal for suppressing the SP loss in a strong cavity satisfying the λ/4 interference condition has been made yet.